Same thing happens to me on xbox. I can't select the hero I played last when I die, but if i quickly switch to a new hero and then switch back it'll let me spawn. But in an event like say, blaster heroes only, I can't even spawn at all because I can't select a single hero. Its absolute bs and I've kinda quit hvv because of it.
The BUG you're experiencing is probably the BUG covered in this report here: https://answers.ea.com/t5/Bug-Reports/HvV-six-second-timer-broken/m-p/8010864
This BUG is actually related to pressing the button which brings the scoreboard just as you choose a character, which makes the game believe you have no character selected, while the character you supposedly selected gets unselectable. It was even more notorious back in the day because heroes would all become unselectable once all your team had selected their characters, which would then trap you in the selection screen until someone on your team died and went back to selection screen (which would then undo the BUG to you).
The one you mentioned seems to be the new selection BUG which happens upon coming back to selection screen after dying. Sometimes when doing that your previously selected character gets unselectable, and you are forced to select another one. Good workaround you found about quickly selecting another character and then coming back to yours to prevent that. I imagine it may be hard to manage because of how quickly you spawn after selection. All that is covered on that report I linked above.
Last thing about the BUG I covered here is that I don't know if it's still in the game. DICE never bothered to state it was fixed or acknowledged, nor I bothered to test it again yet to check for myself, as there are many times DICE fixes something but doesn't put it into the patch notes.
